Types of wireless networks include infrastructure-based wireless networks and ad hoc wireless networks.
Ad hoc networks are self-forming networks which can operate in the absence of any fixed infrastructure, and in some cases the ad hoc network is formed entirely of mobile nodes. A mobile ad hoc network (MANET) typically includes a number of geographically-distributed, potentially mobile units, sometimes referred to as “nodes,” which are wirelessly connected to each other by one or more links (e.g., radio frequency communication channels). The nodes can communicate with each other over a wireless media without the support of an infrastructure-based or wired network. Links or connections between these nodes can change dynamically in an arbitrary manner as existing nodes move within the ad hoc network, as new nodes join or enter the ad hoc network, or as existing nodes leave or exit the ad hoc network. Because the topology of an ad hoc network can change significantly, techniques are needed which can allow the ad hoc network to dynamically adjust to these changes. Due to the lack of a central controller, many network-controlling functions can be distributed among the nodes such that the nodes can self-organize and reconfigure in response to topology changes.
One characteristic of ad hoc network nodes is that each node can directly communicate over a short range with nodes which are a single “hop” away. Such nodes are sometimes referred to as “neighbor nodes.” When a node transmits packets to a destination node and the nodes are separated by more than one hop (e.g., the distance between two nodes exceeds the radio transmission range of the nodes, or a physical barrier is present between the nodes), the packets can be relayed via intermediate relay nodes (“multi-hopping”) along a route until the packets reach the destination node. In such situations, each intermediate relay node routes the packets (e.g., data and control information) to the next node along the route, until the packets reach their final destination. For relaying packets to the next node, each node maintains routing information collected through communications with its neighboring nodes. The routing information can also be periodically broadcast in the network to reflect the current network topology. Alternatively, to reduce the amount of information transmitted for maintaining accurate routing information, the network nodes may exchange routing information only when it is needed. In many multi-hop ad hoc networks, multiple routes can be present between a source node and a destination node for communication of a particular data stream or “flow.”
Ad hoc networks can generally be categorized into two different system architectures which support different control and/or resource allocation protocols. In distributed ad hoc network architectures, each node shares the entire spectrum with other nodes, and each node acts independently in selection of resources (e.g., frequency, time or code allocations). By contrast, in cluster ad hoc network architectures, a clusterhead node acts as a centralized point of control and manages resource allocation, control, and management functions for a cluster of proximate nodes somewhat like a base station in conventional cellular networks. Among other functions, the clusterhead node can direct control information and data traffic to appropriate nodes in the network. Adjacent clusters share resources, such as, time, frequency, or code allocations. The clusterhead node allocates resources among proximate nodes (associated with the clusterhead node) in its cluster based on service requests from the proximate nodes.
Quality-Of-Service (QoS) is becoming an increasingly important issue in many types of MANETs. QoS generally refers to control mechanisms to ensure that connections are able to meet their minimum communication requirements, such as time to provide service, voice quality, minimum data throughput, maximum end-to-end delay, echo, packet loss, reliability and so on. QoS can provide different priority to different users or data flows, or guarantee a certain level of performance to a data flow in accordance with requests from an application program or the internet service provider policy. QoS guarantees are important especially for real-time streaming multimedia applications, for example voice-over-IP (VoIP) and IP-TV, since these types of applications often require fixed bit rates and are delay sensitive, and QoS guarantees must be provided even as the MANET becomes congested.
One “building block” required to implement QoS is commonly referred to as “admission control.” Admission control techniques control admission or entrance of new, inelastic communication streams or “traffic” into a MANET. Admission control can be employed whenever a system has finite capacity. The basic principle of admission control is that a new communication stream should only be “admitted” in the system if it does not cause the system to operate above its maximum capacity level; otherwise, the new communication stream should be denied access to the system. If admission control techniques are not implemented and a new communication stream is admitted causing the system to operate above its capacity level, one or more of the existing connections may no longer be able to support Quality-of-Service (QoS) requirements of the particular data stream.
It is desirable to apply admission control procedures in MANETs such that a new communication stream be permitted to enter the MANET (or be “admitted” to the MANET) only if the existing communication streams are able to maintain their QoS requirements. However, in a MANET, it is difficult to accurately determine the capacity of a particular communication scenario or topology due to the lack of central controller and due to the lack of knowledge of interference conditions in the MANET. Therefore, it is difficult for nodes to individually determine whether a new communication stream will cause one or more existing communication streams to no longer support its QoS requirements. It is desirable that nodes cooperate to perform admission control functions in a distributed manner.
Given that the capacity of a MANET depends on its topology, if a communication stream is denied access to the system, it may retry access through a different route, which may not cause degradation in the QoS of existing communication streams. In other words, upper layers interpret a denial decision as a signal to try alternative routes.
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